OK, so I had another accident, my camera bag toppled of the table at home and my Minolta Dynax 7 hit the ground with my relatively heavy and big Sigma 28-70 F2.8 EX attached to it. Opened the bag, to see it still attached to the camera body, however at a rather odd angle... 
Now some of you who are not particular fond of Sigma will say: well deserved, throw it out... but I actually have good experiences with this particular lens and I am not really in a financial position for big new sales or expensive repairs at the moment.
So, since it is an older lens too, I decided to give a go and at least see what was broken inside. Fearing the worst, I started disassembling the lens from lens mount down, unscrewing stuff and carefully noting how it was assembled.
It turned out to be easier than expected. Maybe this is also due to Sigma being a general manufacturer, creating lenses for different mounts and brands of cameras, but the lens mount unit is almost separate from the true lens unit. There is no direct and "fixed" connection between the lens mount and the lens unit. For example, the mechanical parts moving aperture and AF, that are transmitted from the camera body, are being moved in the lens by a lever and small axle that are not physically connected to the true lens assembly. Meaning any forces exerted on the lens mount in case of an accident like mine, will not damage the lens assembly itself. In fact, the AF axle falls right out of the lens when disassembled, it is only held in place by the screwed on lens mount.
I removed a couple of parts, including the true metal lens mount, to end at what seemed to be the culprit, a slightly bend and broken plastic ring. No further damage was visible, and moving the aperture and focus levers and gear stuff, seemed to indicate the lens to be OK.
The "ring" broken, was actually broken at a seemingly insignificant place, at a point where it narrowed from a wide strong ring to a minor 3 mm wide plastic "bridge". As a consequence of the forces exerted during the fall, the thick part of the ring had bend though, and now stuck out about 1 mm at one end. Trying to bend it back proved difficult, as at this part of the "ring", the ring was reinforced at the back with a kind of "tube" like extension, going about 7mm deeper into the camera.
I managed to bend it back some, and reassembled the lens completely, to see if it would actually still work. Although it was way better, the lens still was at a minor odd angle to the camera body, but I could at least test it now. AF and aperture seemed fine!
I than disassembled the whole unit a second time, trying to assess what could be done. I tried to remove the affected part by unscrewing three screws, but it seems this part is also glued in the lens assembly. So far for the option of removing it and sending it of to Sigma to ask for a replacement...
I than decided to have a look if I could somehow bend the ring back and fix it. After some difficulties, I finally managed to bend it back some, and fix it in place with super-glue. It seemed right, but after reassembling the lens, it turned out to be still at a very minor odd angle to the mount. Just maybe a 0.2 to 0.3 mm gap at one side.
I disassembled again, and noticed the other parts of the lens mount fitted onto the damaged ring using screws that fitted into tiny holes that were slightly raised compared to the rest of the "ring". Since it was just plastic, I took a piece of sand paper and started to take off some of the plastic of one of the three screw points at the point of breakage. When it seemed right, I reassembled the whole lens.
Bingo! It now seems to be OK. Everything works fine, and the lens no longer stands at an odd angle...
For the moment, it will have to do. Everything feels OK at least, no strange hiccups of parts indicating further issues. I think the breakage and bending of this plastic part took the main blow and forces off of the rest of the lens assembly.
Anyway, one last question to you all. Of course, although it now visually seems OK, there may still be a very minor deviation from perpendicular. I started wondering how big an issue this would be? I have no idea how much "depth-of-field" there actually is at different apertures on the film plane in the camera. Will any minor deviation immediately show up? Or is there actually some "tolerance" in depth of field..., even for the biggest F2.8 aperture setting?
I have been thinking of photographing a newspaper to see how the sharpness is across the film plane. Of course, this would require very careful setup, as I could just as well screw it up there.
But anyone who can comment on this or has suggestions?
Marco
Now some of you who are not particular fond of Sigma will say: well deserved, throw it out... but I actually have good experiences with this particular lens and I am not really in a financial position for big new sales or expensive repairs at the moment.
So, since it is an older lens too, I decided to give a go and at least see what was broken inside. Fearing the worst, I started disassembling the lens from lens mount down, unscrewing stuff and carefully noting how it was assembled.
It turned out to be easier than expected. Maybe this is also due to Sigma being a general manufacturer, creating lenses for different mounts and brands of cameras, but the lens mount unit is almost separate from the true lens unit. There is no direct and "fixed" connection between the lens mount and the lens unit. For example, the mechanical parts moving aperture and AF, that are transmitted from the camera body, are being moved in the lens by a lever and small axle that are not physically connected to the true lens assembly. Meaning any forces exerted on the lens mount in case of an accident like mine, will not damage the lens assembly itself. In fact, the AF axle falls right out of the lens when disassembled, it is only held in place by the screwed on lens mount.
I removed a couple of parts, including the true metal lens mount, to end at what seemed to be the culprit, a slightly bend and broken plastic ring. No further damage was visible, and moving the aperture and focus levers and gear stuff, seemed to indicate the lens to be OK.
The "ring" broken, was actually broken at a seemingly insignificant place, at a point where it narrowed from a wide strong ring to a minor 3 mm wide plastic "bridge". As a consequence of the forces exerted during the fall, the thick part of the ring had bend though, and now stuck out about 1 mm at one end. Trying to bend it back proved difficult, as at this part of the "ring", the ring was reinforced at the back with a kind of "tube" like extension, going about 7mm deeper into the camera.
I managed to bend it back some, and reassembled the lens completely, to see if it would actually still work. Although it was way better, the lens still was at a minor odd angle to the camera body, but I could at least test it now. AF and aperture seemed fine!
I than disassembled the whole unit a second time, trying to assess what could be done. I tried to remove the affected part by unscrewing three screws, but it seems this part is also glued in the lens assembly. So far for the option of removing it and sending it of to Sigma to ask for a replacement...
I than decided to have a look if I could somehow bend the ring back and fix it. After some difficulties, I finally managed to bend it back some, and fix it in place with super-glue. It seemed right, but after reassembling the lens, it turned out to be still at a very minor odd angle to the mount. Just maybe a 0.2 to 0.3 mm gap at one side.
I disassembled again, and noticed the other parts of the lens mount fitted onto the damaged ring using screws that fitted into tiny holes that were slightly raised compared to the rest of the "ring". Since it was just plastic, I took a piece of sand paper and started to take off some of the plastic of one of the three screw points at the point of breakage. When it seemed right, I reassembled the whole lens.
Bingo! It now seems to be OK. Everything works fine, and the lens no longer stands at an odd angle...
For the moment, it will have to do. Everything feels OK at least, no strange hiccups of parts indicating further issues. I think the breakage and bending of this plastic part took the main blow and forces off of the rest of the lens assembly.
Anyway, one last question to you all. Of course, although it now visually seems OK, there may still be a very minor deviation from perpendicular. I started wondering how big an issue this would be? I have no idea how much "depth-of-field" there actually is at different apertures on the film plane in the camera. Will any minor deviation immediately show up? Or is there actually some "tolerance" in depth of field..., even for the biggest F2.8 aperture setting?
I have been thinking of photographing a newspaper to see how the sharpness is across the film plane. Of course, this would require very careful setup, as I could just as well screw it up there.
But anyone who can comment on this or has suggestions?
Marco
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... good overall sharpness. Actually, I had more trouble keeping the negatives flat in my glassless negative carrier, causing some blurring on a quick test print, but the final prints showed no apparent issues.
